1. Field of the Invention
The present invention relates to a process for preparing high-purity cyclopropyl methyl ketone (CPMK) from compositions, e.g., reaction discharges or mixtures, that contain CPMK, 4,5-dihydro-2-methylfaran (DHMF), addition products of DHMF, and, optionally, impurities which are produced in the synthesis of CPMK from 2-acetylbutyrolactone (ABL).
2. Description of the Background
CPMK is a valuable intermediate which is required in high purity for the synthesis of a variety of agricultural and pharmaceutical active compounds. A purity of &gt;99% by weight is required for the use of CPMK as a building block for synthesis in the agricultural and pharmaceutical sectors.
CPMK can be particularly advantageously prepared from ABL by cyclizing ABL to give CPMK with elimination of CO.sub.2 under alkali metal halide catalysis [Chemistry Letters 11, pp. 1149-52 (1975)]. It may be prepared continuously by introducing the alkali metal halide in a solvent at high temperature and continuously adding the ABL. Since, in the reaction, in addition to the main product CPMK, DHMF is also formed at a content of usually 5 to 30% by weight, as product of value of the reaction, a mixture of CPMK and DHMF is always continuously distilled off simultaneously from the reaction. Corresponding preparation processes are disclosed, for example, in EP-A 0 552 586 or DE-A 195 03 241. However, the CPMK obtained in this manner does not comply with the desired purity requirements because of the unacceptably high DHMF content.
DHMF is sufficiently more volatile than CPMK and can therefore in principle be separated off from CPMK by distillation without problems, in order to obtain CPMK of high purity. However, in the presence of acid, DHMF reacts with numerous nucleophiles, such as water, alcohols, amines etc., to give high-boiling addition products (cf Houben/Weyl: Methoden der organischen Chemie [Methods in Organic Chemistry], Vol. VI/3, 1965, p. 698, incorporated herein by reference). The addition of water to DHMF forms, for example, acetopropanol, which itself can in turn add to DHMF. These addition reactions are reversible, so that at high temperatures acetopropanol or its addition product to DHMF or other addition products to DHMF can release DHMF again by elimination or cyclization.
In a crude mixture of CPMK and DHMF, small amounts of water or other nucleophilic impurities are also always present, which reversibly convert DHMF into high-boilers in the manner described above. Purification of CPMK by distillation to separate off DHMF, therefore leads, depending on the bottom temperature, to recleavage of the higher-boiling DHMF addition products and thus again to contamination of CPMK with DHMF in the final distillation fraction. In the purification of CPMK by distillation, there is therefore always the problem of contamination of the CPMK by recleavage of the addition products of DHMF as a function of the bottom temperature. Because of the bottom temperatures required in conventional distillations, e.g., 110 to 180.degree. C., which are necessary to comply with the purity requirements, there is the necessity of separating off further DHMF from the first runnings and final fractions of the distillation. However, this requires a high technical complexity and reduces the CPMK yield.
Lowering the bottom temperature by applying a vacuum and thus preventing recleavage of DHMF addition products and the thus ensuing contamination of CPMK can be accomplished. However, the technical complexity for condensing the relatively low-boiling CPMK is too great, making this process unfeasible.